Powdery Mildew Resistance In Cucumber Research Articles

A genome-wide association study reveals molecular mechanism underlying powdery mildew resistance in cucumber

BackgroundPowdery mildew is a disease with one of the most substantial impacts on cucumber production globally. The most efficient approach for controlling powdery mildew is the development of genetic resistance; however, few genes associated with inherent variations in cucumber powdery mildew resistance have been identified as of yet.ResultsIn this study, we re-sequence 299 cucumber accessions, which are divided into four geographical groups. A genome-wide association study identifies 50 sites significantly associated with natural variations in powdery mildew resistance. Linkage disequilibrium analysis further divides these 50 sites into 32 linkage disequilibrium blocks containing 41 putative genes. Virus-induced gene silencing and gene expression analysis implicate CsGy5G015960, which encodes a phosphate transporter, as the candidate gene regulating powdery mildew resistance. On the basis of the resequencing data, we generate five CsGy5G015960 haplotypes, identifying Hap.1 as the haplotype most likely associated with powdery mildew resistance. In addition, we determine that a 29-bp InDel in the 3′ untranslated region of CsGy5G015960 is responsible for mRNA stability. Overexpression of CsGy5G015960Hap.1 in the susceptible line enhances powdery mildew resistance and phosphorus accumulation. Further comparative RNA-seq analysis demonstrates that CsGy5G015960Hap.1 may regulate cucumber powdery mildew resistance by maintaining a higher H2O2 level through the depletion of multiple class III peroxidases.ConclusionsHere we identify a candidate powdery mildew-resistant gene in cucumber using GWAS. The identified gene may be a promising target for molecular breeding and genetic engineering in cucumber to enhance powdery mildew resistance.

You don't win friends with bad salad! A gene editing approach to enhance the powdery mildew resistance in cucumber.

You don't win friends with bad salad! A gene editing approach to enhance the powdery mildew resistance in cucumber.

CsPM5.2, a phosphate transporter protein-like gene, promotes powdery mildew resistance in cucumber.

Powdery mildew (PM) is one of the most serious fungal diseases affecting cucumbers (Cucumis sativus L.). The mechanism of PM resistance in cucumber is intricate and remains fragmentary as it is controlled by several genes. In this study, we detected the major-effect Quantitative Trait Locus (QTL), PM5.2, involved in PM resistance by QTL mapping. Through fine mapping, the dominant PM resistance gene, CsPM5.2, was cloned and its function was confirmed by transgenic complementation and natural variation identification. In cultivar 9930, a dysfunctional CsPM5.2 mutant resulted from a single nucleotide polymorphism in the coding region and endowed susceptibility to PM. CsPM5.2 encodes a phosphate transporter-like protein PHO1; H3. The expression of CsPM5.2 is ubiquitous and induced by the PM pathogen. In cucumber, both CsPM5.2 and Cspm5.1 (Csmlo1) are required for PM resistance. Transcriptome analysis suggested that the salicylic acid (SA) pathway may play an important role in CsPM5.2-mediated PM resistance. Our findings help parse the mechanisms of PM resistance and provide strategies for breeding PM-resistant cucumber cultivars.

A single-nucleotide substitution in the leucine-rich-repeat-only gene CsLRR1 confers powdery mildew resistance in cucumber

A single-nucleotide substitution in the leucine-rich-repeat-only gene CsLRR1 confers powdery mildew resistance in cucumber

Development of powdery mildew (Podosphaera xanthii) resistance in cucumber (Cucumis sativus) using CRISPR/Cas9-mediated mutagenesis of CsaMLO8.

Powdery mildew (PM) diseases may severely limit the production of various crops, including members of to the Cucurbitaceae family. Successful PM infection relies on the Mildew Resistance Locus O (MLO) plant gene family, which encodes susceptibility (S) factors essential for fungus penetration into the host cell. In cucumber, natural mutations in CsaMLO8 confer resistance to the PM pathogen Podosphaera xanthii. Here, we used CRISPR/Cas9-mediated mutagenesis to generate PM resistance in the susceptible cucumber cv. 'Ilan'. Two transgene-free Csamlo8 CRISPR mutant lines (Csamlo-cr-1 and Csamlo-cr-2) were isolated, the first with a 5 bp deletion in exon 1, and the second harboring a 1280 bp deletion and 10 bp insertion between exons 1 and 5. Both lines showed high resistance to PM under semi-commercial growth conditions in the summer growing seasons of 2019 and 2021. These results provide the basis for generating transgene-free powdery mildew resistance in cucumber in any genetic background. This method can directly be employed on commercial cultivars and hybrid parental lines, and thereby substantially shorten and simplify the breeding process for PM resistance in cucumber.

Evaluation of resistant genotypes and their characterization using molecular markers linked for powdery mildew resistance in cucumber (Cucumis sativus L.)

AbstractPowdery mildew (PM) is one of the most severe fungal diseases of cucumber that limits its production worldwide. In this study, 140 genotypes of cucumber were screened for disease resistance under field and artificial conditions, and then validated with eight known SSR markers linked to PM resistance. Among these genotypes, genotype GS140 was found resistant (R), whereas GS148, GS16 and GS70 were moderately resistant, and GS169 was found to be tolerant. Of all the eight markers, only C31, C80, C162, SSR16472 and SSR16881 amplified the reported linked allele. The 127 bp allele of SSR16881 was found to be associated with the lowest disease severity of 37.65%. The associated markers could further be verified for their usability using linkage studies and the contrast genotypes in the present study could serve as a tool for selection in future marker-assisted selection breeding strategies for PM resistance.

Characterization of lncRNAs and mRNAs Involved in Powdery Mildew Resistance in Cucumber.

Powdery mildew (PM) is a severe fungal disease of cucumber worldwide. Identification of genetic factors resistant to PM is of great importance for marker-assisted breeding to ensure cucumber production. Long noncoding RNAs (lncRNAs) and microRNAs (miRNAs) have been shown to play important roles in plant development and immunity; however, whether they have a role in PM response in cucurbit crops remains unknown. We performed strand-specific RNA sequencing and miRNA sequencing using RNA from cucumber leaves of two near-isogenic lines (NILs), S1003 and NIL (Pm5.1) infected with PM, and systematically characterized the profiles of cucumber lncRNAs and messenger RNA (mRNAs) responsive to PM. In total, we identified 12,903 lncRNAs and 25,598 mRNAs responsive to PM. Differential expression (DE) analysis showed that 119 lncRNAs and 136 mRNAs correlated with PM resistance. Functional analysis of these DE lncRNAs and DE mRNAs revealed that they are significantly associated with phenylpropanoid biosynthesis, phenylalanine metabolism, ubiquinone and other terpenoid-quinone biosynthesis, and endocytosis. Particularly, two lncRNAs, LNC_006805 and LNC_012667, might play important roles in PM resistance. In addition, we also predicted mature miRNAs and competing endogenous RNA (ceRNA) networks of lncRNA-miRNA-mRNA involved in PM resistance. A total of 49 DE lncRNAs could potentially act as target mimics for 106 miRNAs. Taken together, our results provide an abundant resource for further exploration of cucumber lncRNAs, mRNAs, miRNAs, and ceRNAs in PM resistance, and will facilitate the molecular breeding for PM-resistant varieties to control this severe disease in cucumber.

QTL-seq analysis of powdery mildew resistance in a Korean cucumber inbred line.

QTL mapping and RT-PCR analyses identified the CsGy5G015660 as a strong powdery mildew resistance candidate gene and natural variation of CsGy5G015660 allele was observed using 115 core germplasm. Powdery mildew (PM) is among the most serious fungal diseases encountered in the cultivation of cucurbits. The development of PM-resistant inbred lines is thus of considerable significance for cucumber breeding programs. In this study, we applied bulked segregant analysis combined with QTL-seq to identify PM resistance loci using F2 population derived from a cross between two Korean cucumber inbred lines, PM-R (resistant) and PM-S (susceptible). Genome-wide SNP profiling using bulks of the two extreme phenotypes identified two QTLs on chromosomes 5 and 6, designated pm5.2 and pm6.1, respectively. The two PM resistance loci were validated using molecular marker-based classical QTL analysis: pm5.2 (30% R2 at LOD 11) and pm6.1 (11% R2 at LOD 3.2). Furthermore, reverse transcriptase-PCR analyses, using genes found to be polymorphic between PM-R and PM-S, were conducted to identify the candidate gene(s) responsible for PM resistance. We found that transcripts of the gene CsGy5G015660, encoding a putative leucine-rich repeat receptor-like serine/threonine-protein kinase (RPK2), showed specific accumulation in PM-R prior to the appearance of disease symptoms, and was accordingly considered a strong candidate gene for PM resistance. In addition, cleaved amplified polymorphic sequence markers from CsGy5G015660 were developed and used to screen 35 inbred lines. Natural variation in the CsGy5G015660 allele was also observed based on analysis of a core collection of 115 cucumber accessions. Our results provide new genetic insights for gaining a better understanding of the genetic basis of PM resistance in cucumber, and pave the way for further utilization in cucumber PM resistance breeding programs.

Whole-Genome Resequencing of a Cucumber Chromosome Segment Substitution Line and Its Recurrent Parent to Identify Candidate Genes Governing Powdery Mildew Resistance.

Cucumber is an economically important vegetable crop worldwide. Powdery mildew (PM) is one of the most severe diseases that can affect cucumber crops. There have been several research efforts to isolate PM resistance genes for breeding PM-resistant cucumber. In the present study, we used a chromosome segment substitution line, SSL508-28, which carried PM resistance genes from the donor parent, JIN5-508, through twelve generations of backcrossing with a PM-susceptible inbred line, D8. We performed whole-genome resequencing of SSL508-28 and D8 to identify single nucleotide polymorphisms (SNPs), and insertions and deletions (indels). When compared against the reference genome of the inbred cucumber line 9930, a total of 468,616 SNPs and 67,259 indels were identified in SSL508-28, and 537,352 SNPs and 91,698 indels were identified in D8. Of these, 3,014 non-synonymous SNPs and 226 frameshift indels in SSL508-28, and 3,104 non-synonymous SNPs and 251 frameshift indels in D8, were identified. Bioinformatics analysis of these variations revealed a total of 15,682 SNPs and 6,262 indels between SSL508-28 and D8, among which 120 non-synonymous SNPs and 30 frameshift indels in 94 genes were detected between SSL508-28 and D8. Finally, out of these 94 genes, five resistance genes with nucleotide-binding sites and leucine-rich repeat domains were selected for qRT-PCR analysis. This revealed an upregulation of two transcripts, Csa2M435460.1 and Csa5M579560.1, in SSL508-28. Furthermore, the results of qRT-PCR analysis of these two genes in ten PM resistant and ten PM susceptible cucumber lines showed that when exposed to PM, Csa2M435460.1 and Csa5M579560.1 exhibited a higher expression level of resistant lines than susceptible lines. This indicates that Csa2M435460.1 and Csa5M579560.1 are candidate genes for PM resistance in cucumber. In addition, the non-synonymous SNPs in Csa2M435460.1 and Csa5M579560.1, identified in SSL508-28 and D8, might be the key to high PM-resistance in SSL508-28.

Loss-of-Function Mutations in CsMLO1 Confer Durable Powdery Mildew Resistance in Cucumber (Cucumis sativus L.).

Powdery mildew (PM) is a serious fungal disease of cucumber worldwide. The identification of resistance genes is very important for resistance breeding to ensure cucumber production. Here, natural loss-of-function mutations at an MLO homologous locus, CsMLO1, were found to confer durable PM resistance in cucumber. CsMLO1 encoded a cell membrane protein, was mainly expressed in leaves and cotyledons, and was up-regulated by PM at the early stage of host–pathogen interaction. Ectopic expression of CsMLO1 rescued the phenotype of the PM resistant Atmlo2 Atmlo12 double mutant to PM susceptible in Arabidopsis. Domesticated and wild resistant cucumbers originating from various geographical regions of the world were found to harbor three independent natural mutations that resulted in CsMLO1 loss of function. In addition, between the near-isogenic lines (NILs) of PM resistant and susceptible, S1003 and NIL(Pm5.1), quantitative RT-PCR revealed that there is no difference at expression levels of several genes in the pathways of ethylene, jasmonic acid or salicylic acid. Moreover, the two NILs were used for transcriptome profiling to explore the mechanism underlying the resistance. Several genes correlated with plant cell wall thickening are possibly involved in the PM resistance. This study revealed that loss of function of CsMLO1 conferred durable PM resistance, and that this loss of function is necessary but alone may not be sufficient for PM resistance in cucumber. These findings will facilitate the molecular breeding of PM resistant varieties to control this destructive disease in cucumber.

Fine mapping of a dominantly inherited powdery mildew resistance major-effect QTL, Pm1.1, in cucumber identifies a 41.1 kb region containing two tandemly arrayed cysteine-rich receptor-like protein kinase genes.

A dominantly inherited major-effect QTL for powdery mildew resistance in cucumber was fine mapped. Two tandemly arrayed cysteine-rich receptor-like protein kinase genes were identified as the most possible candidates. Powdery mildew (PM) is one of the most severe fungal diseases of cucumber (Cucumis sativus L.) and other cucurbit crops, but the molecular genetic mechanisms of powdery mildew resistance in cucurbits are still poorly understood. In this study, through marker-assisted backcrossing with an elite cucumber inbred line, D8 (PM susceptible), we developed a single-segment substitution line, SSSL0.7, carrying 95 kb fragment from PM resistance donor, Jin5-508, that was defined by two microsatellite markers, SSR16472 and SSR16881. A segregating population with 3600 F2 plants was developed from the SSSL0.7 × D8 mating; segregation analysis confirmed a dominantly inherited major-effect QTL, Pm1.1 in cucumber chromosome 1 underlying PM resistance in SSSL0.7. New molecular markers were developed through exploring the next generation resequenced genomes of Jin5-508 and D8. Linkage analysis and QTL mapping in a subset of the F2 plants delimited the Pm1.1 locus into a 41.1 kb region, in which eight genes were predicted. Comparative gene expression analysis revealed that two concatenated genes, Csa1M064780 and Csa1M064790 encoding the same function of a cysteine-rich receptor-like protein kinase, were the most likely candidate genes. GFP fusion protein-aided subcellular localization indicated that both candidate genes were located in the plasma membrane, but Csa1M064780 was also found in the nucleus. This is the first report of dominantly inherited PM resistance in cucumber. Results of this study will provide new insights into understanding the phenotypic and genetic mechanisms of PM resistance in cucumber. This work should also facilitate marker-assisted selection in cucumber breeding for PM resistance.

Mining candidate genes associated with powdery mildew resistance in cucumber via super-BSA by specific length amplified fragment (SLAF) sequencing.

BackgroundPowdery mildew (PM) is the most common fungal disease of cucumber and other cucurbit crops, while breeding the PM-resistant materials is the effective way to defense this disease, and the recent development of modern genetics and genomics make us aware of that studying the resistance genes is the essential way to breed the PM high-resistance plant. With the ever increasing throughput of next-generation sequencing (NGS), the development of specific length amplified fragment sequencing (SLAF-seq) as a high-resolution strategy for large-scale de novo SNP discovery is gradually applied for functional gene mining. Here we combined the bulked segregant analysis (BSA) with SLAF-seq to identify candidate genes associated with PM resistance in cucumber.MethodsA segregating population comprising 251 F2 individuals was developed using H136 (female parent) as susceptible parent and BK2 (male parent) as resistance donor. After PMR test, total genomic DNA was prepared from each plant. Systemic genomic analysis of the GC content, repeat sequence, etc. was carried out by prediction software SLAF_Predict to establish condition to ensure the uniformity and density of the molecular markers. After samples were gel purified, SLAFs were generated at Biomarker Technologies Corporation in Beijing. Based on SLAF tags and the PMR test result, the hot region were annotated.ResultsA total of 73,100 high-quality SLAF tags with an average depth of 99.11× were sequenced. Among these, 5,355 polymorphic tags were identified with a polymorphism rate of 7.34 %, including 7.09 % SNPs and other polymorphism types. Finally, 140 associated SLAFs were identified, and two main Hot Regions were detected on chromosome 1 and 6, which contained five genes invovled in defense response, toxin metabolism, cell stress response, and injury response in cucumber.ConclusionsAssociated markers identified by super-BSA in this study, could not only speed up the study of the PMR genes, but also provide a feasible solution for breeding the marker-assisted PMR cucumber. Moreover, this study could also be extended to any other species with reference genome.

Identification and fine mapping of pm5.1: a recessive gene for powdery mildew resistance in cucumber (Cucumis sativus L.)

Powdery mildew is one of the most important diseases of cucumber (Cucumis sativus L.). For many years, researchers have studied and attempted to isolate resistance genes for this disease for breeding powdery mildew-resistant cucumber. In this study, bulked segregant analysis and an initial quantitative trait loci (QTLs) analysis based on a local linkage map were performed using an F2 population. A major locus on the long arm of chromosome 5 was identified for powdery mildew resistance and named pm5.1. For fine mapping of the pm5.1 locus, BC3F1 and BC2F2 populations that segregated only at the target genomic region were developed by marker-assisted selection. The location of the pm5.1 locus was confirmed by QTL analysis using a subset of the BC3F1 population and was delimited in a 1.8-cM interval between flanking markers UW065011 and UW065190. Using the large BC3F1 (n = 480) and BC2F2 (n = 483) populations, pm5.1 was mapped to a ~170-kb region between markers UW065021 and UW065094. Based on the annotations for the genes in this region, a MLO-like gene was identified as the most likely candidate for the pm5.1. Sequence alignment analysis of the MLO-like genes from susceptible and resistant cucumber lines revealed two types of mutations in this candidate gene that may result in recessive resistance to powdery mildew in the resistant cucumber lines.

QTL mapping of powdery mildew resistance in WI 2757 cucumber (Cucumis sativus L.)

Powdery mildew (PM) is a very important disease of cucumber (CucumissativusL.). Resistant cultivars have been deployed in production for a long time, but the genetic mechanisms of PM resistance in cucumber are not well understood. A 3-year QTL mapping study of PM resistance was conducted with 132 F2:3 families derived from two cucumber inbred lines WI 2757 (resistant) and True Lemon (susceptible). A genetic map covering 610.4cM in seven linkage groups was developed with 240 SSR marker loci. Multiple QTL mapping analysis of molecular marker data and disease index of the hypocotyl, cotyledon and true leaf for responses to PM inoculation identified six genomic regions in four chromosomes harboring QTL for PM resistance in WI 2757. Among the six QTL, pm1.1 and pm1.2 in chromosome 1 conferred leaf resistance. Minor QTL pm3.1 (chromosome 3) and pm4.1 (chromosome 4) contributed to disease susceptibility. The two major QTL, pm5.1 and pm5.2 were located in an interval of ~40cM in chromosome 5 with each explaining 21.0-74.5% phenotypic variations. Data presented herein support two recessively inherited, linked major QTL in chromosome 5 plus minor QTL in other chromosomes that control the PM resistance in WI 2757. The QTL pm5.2 for hypocotyl resistance plays the most important role in host resistance. Multiple observations in the same year revealed the importance of scoring time in the detection of PM resistance QTL. Results of this study provided new insights into phenotypic and genetic mechanisms of powdery mildew resistance in cucumber.

Identification of differentially expressed genes between powdery mildew resistant near-isogenic line and susceptible line of cucumber by suppression subtractive hybridization

A population of BC 7 near-isogenic cucumber line (NIL210) was made from a cross between the powdery mildew susceptible cultivar (D8) and resistant cultivar (JIN5-508) in order to get powdery mildew resistance genes in cucumber. In this study, suppression subtractive hybridizations (SSHs) were applied to identify differential expressed genes between NIL210 and D8. 166 differentially expressed ESTs were identified from two SSH cDNA libraries including constitutively expressed resistant and susceptible factors. Most of the classified genes belong to four pathogenesis-related categories: ‘disease/defense’, ‘signal transduction’, ‘transcription’ and ‘transporter’, which altogether accounted for 46–53% of the genes classified. Some candidate powdery mildew resistant genes were also identified, such as GT053536 (TIR-NBS-LRR protein) and GT053570 (phenylalanine ammonia-lyase 2). These results will contribute to the better understanding of molecular mechanisms underlying powdery mildew resistance in cucumber.

QTL molecular marker location of powdery mildew resistance in cucumber (Cucumis sativus L.)

The cucumber lines, S94 (Northern China open-field type, powdery mildew (PM) susceptible) and S06 (European greenhouse type, PM resistant), and their F(6:7) populations were used to investigate PM resistance under seedling spray inoculation in 2005/Autumn and 2006/Spring. QTL analysis was undertaken based on a constructed molecular linkage map of the corresponding F(6) population using composite interval mapping. A total of four QTLs (pm1.1, pm2.1, pm4.1 and pm6.1) for PM resistance were identified and located on LG 1, 2, 4 and 6, respectively, explaining 5.2%-21.0% of the phenotypic variation. Three consistent QTLs (pm1.1, pm2.1 and pm4.1) were detected under the two test conditions. The QTL pm6.1 was only identified in 2005/Autumn. The total phenotypic variation explained by the QTLs was 52.0% and 42.0% in 2005/Autumn and 2006/Spring, respectively. Anchor markers tightly linked to those loci (<5 cM) could lay a basis for both molecular marker-assisted breeding and map-based gene cloning of the PM-resistance gene in cucumber.

Inheritance of Powdery Mildew Resistance in Cucumber ( Cucumis sativus L.) and Development of an AFLP Marker for Resistance Detection

Cucumber powdery mildew is one of the most destructive diseases of cucumber throughout the world. In the present study, inheritance of powdery mildew resistance in three crosses, and linkage of resistance with amplified fragment length polymorphism (AFLP) markers are studied to formulate efficient strategies for breeding cultivars resistant to powdery mildew. The joint analysis of multiple generations and AFLP technique has been applied in this study. The best model is the one with two major genes, additive, dominant, and epistatic effects, plus polygenes with additive, dominant, and epistatic effects (E-1-0 model). The heritabilities of the major genes varied from 64.26% to 97.82%, and susceptibility was incompletely dominant for the two major genes in the three crosses studied. The additive effects of the two major genes and the dominant effect of the second major gene were high, and the epistatic effect of the additive-dominant between the two major genes was the highest in cross I. In cross II, the absolute value of the additive effect, dominant effect, and potential ratio of the first major gene were far higher than those of the second major gene, and the epistatic effect of the additive-additive was the highest. The genetic parameters of the two major genes in cross III were similar to those in cross II. Correlation and regression analyses showed that marker E25/M63-103 was linked to a susceptible gene controlling powdery mildew resistance. The marker could account for 19.98% of the phenotypic variation. When the marker was tested on a diverse set of 29 cucumber lines, the correlation between phenotype and genotype was not significant, which suggested cultivar specialty of gene expression or different methods of resistance to powdery mildew. The target DNA fragment was 103 bp in length, and only a small part was found to be homologous to DNA in the other species evaluated, which indicated that it was unique to the cucumber genome.

QTL analysis of powdery mildew resistance in cucumber (Cucumis sativus L.)

QTL analysis of powdery mildew resistance in cucumber (Cucumis sativus L.)

QTL analysis of powdery mildew resistance in cucumber (Cucumis sativus L.)

A population of F7 recombinant inbred lines (RILs) was made from a cross between susceptible ('Santou') and resistant (PI197088-1) lines of cucumber in order to study powdery mildew resistance loci. Susceptibility to powdery mildew in the F7 RIL individuals showed a continuous distribution from susceptible to resistant, suggesting that powdery mildew resistance is controlled by quantitative trait loci (QTLs). A QTL analysis identified two and three loci for powdery mildew resistance under 26 and 20 degrees C conditions, respectively. One QTL was found in the same position under both temperature conditions. Therefore, it is more likely that one major QTL acts under both temperature conditions and that other QTLs are specific to the two temperature conditions. The above results suggest that the four QTLs are controlled in a different temperature manner, and that their combination played an important role in expressing a high level of resistance to powdery mildew in this cucumber population. Sequence-tagged site (STS) markers associated with each QTL were developed and would be useful for breeding a cucumber line with a high level of powdery mildew resistance.

Powdery Mildew Resistance in Cucumber

Using an improved method for evaluating powdery mildew (Sphaerotheca cucurbitae) resistance in cucumber, experiments were conducted for the breeding of varieties resistant to powdery mildew that would be adapted to greenhouse cultivation during the period from winter to spring in Japan. The relationship between the resistance to the pathogen and the ambient temperature was analyzed by spray inoculation of a conidial suspension under controlled conditions. Although the varieties. Asomidori-5-gou and Natsufushinari were resistant at temperatures between 25 to 30°C, they became susceptible at 15 to 20°C. The resistance was affected by the duration of the exposure to 30 and 15°C during the day. To screen resistant varieties and lines independently of the temperature, 295 cucumber accessions were inoculated at 20 and 26°C. All the accessions tested could be divided into 3 types based on the differ. ence in the resistance response to temperature. Many of the resistant accessions were Chinese varieties and lines. PI 197088-5, a progeny of PI197088 which originated in India, displayed the highest level of resistance among all the accessions. Thus PI197088-5, whose resistance is temperature-independent appears to be the most suitable parent for the breeding of powdery mildew-resistant cucumber varieties. The inheritance of powdery mildew resistance in PI197088-5 was analyzed using Natsufushinari (resistant under high temperature conditions) and Sharp I (susceptible). As a result it was considered that the powdery mildew resistance in PI197088-5 was due to 2 gene pairs, a recessive gene and an incompletely dominant gene, with the genotype designated as aaBB, while the genotype of Natsufushinari was designated as aabb and that of Sharp 1 as AAbb. The nature of the host-pathogen interaction in PI197088-1 which is the sib-line of PI197088-5 was observed microscopically. The retardation of hyphal growth and development was observed at 2 days after inoculation. Conidia were hardly formed on the epidermis and the number of haustoria in the epidermal cells of PI197088-1 was much lower than that in the susceptible variety Sharp 1. Fluorescence of epidermal cells was observed along the hyphae in PI197088-1 at 6 days after the inoculation, suggesting the existence of a hypersensitive reaction.